HCV is a major health problem with as many as 3.9 million Americans chronically infected. As the population ages, these patients will be increasingly at risk for developing liver disease and hepatocellular carcinoma. The current model for HCV study, the chimpanzee, is too large and too expensive for many studies of HCV replication. An alternative surrogate model for HCV infection is replication of GBV-B in tamarins. GBV-B is closely related to HCV to the degree that the GBV-B protease cleaves an HCV protein substrate. GBV-B is believed to be a tamarin virus that causes an acute, self-limiting hepatitis. The virus can be transmitted via infectious plasma causing a readily reproducible hepatitis. The applicants hypothesize that GBV-B is so closely related to HCV that the study of GBV-B in tamarins and in tamarin hepatocyte cell cultures will provide data on replication, pathogenesis and the immune response that will be useful in understanding HCV infections in humans, and that furthermore this animal model will be of great value in the testing of antiviral drug strategies for HCV. A model system for study of GBV-B in tamarins will be initiated by generating and titering a pool of serum containing GBV-B. The course of GBV-B infections in normal and immunosuppressed tamarins will be monitored by reverse transcriptase PCR (RT-PCR) and ELISA assays being developed for this purpose. The host range of GBV-B will be examined by infecting squirrel, owl, and spider monkeys with infectious GBV-B serum to determine the optimal host for future investigations. A tissue culture system for hepatocytes isolated from naive tamarins will be used for in vitro infectivity studies. Immortalized hepatocyte cell lines will be developed from primary tamarin hepatocytes in an effort to establish continuous hepatocyte cell lines susceptible to GBV-B infection. A preliminary investigation of the humoral immune response to GBV-B infections in tamarins will be undertaken with emphasis on identifying the major viral antigenic regions responsible for eliciting the antibody response. An in vitro neutralization assay will be developed for use in preliminary examination of the requirement of the GBV-B neutralization. GBV-B and GBV-B/HCV hybrid molecular clones will be tested for infectivity following injection into tamarin liver or electroporation into primary hepatocytes or continuous hepatocyte cell lines. It is anticipated that a viable surrogate model for HCV infection will be developed.